Radionuclides that are present in a sample may be identified and quantified by detecting and analyzing the radiation emitted from the sample. This is important in many contexts, such as the detection of tritium, radon, radium, and uranium in drinking water; the detection of strontium in food; the detection of 14C in food, alcohol, and biofuels; evaluations of tritium and 14C emissions from nuclear power plants; the monitoring of radioactivity during decommissioning of nuclear reactors; tracer measurements in oil exploration; adsorption, distribution, metabolism, and excretion (ADME) studies; the detection of radionuclides in biological samples (e.g., identification of viable drug pathways in drug development); and radiocarbon dating of archaeological samples, as well as many other biological and environmental contexts.
There are a variety of systems and analytical techniques available for detection of events caused by the radioactive decay of radionuclides in a sample. Where the sample contains a plurality of radionuclides that emit different kinds of radiation (e.g., both alpha and beta emitters), or where a sample contains a radionuclide of unknown type, it is important to be able to determine whether a detected radioactive event is caused by alpha, beta, or gamma radiation.
For example, in liquid scintillation counting, a sample material containing one or more radionuclides to be identified is mixed with a solvent capable of dissolving the sample material, along with a scintillator (e.g., a fluor). A vial of the resulting cocktail is placed in a detector comprising one or more photomultiplier tubes. When the radionuclide(s) undergo radioactive decay, the emitted decay energy causes excitation of the scintillator and release of UV light, which is detected. The intensity of the light is a function of the decay energy, and the shape of the detected pulse can be used to distinguish between different kinds of radioactive events, e.g., alpha, beta, or gamma radiation. The detector produces a pulse signal corresponding to each of a plurality of radioactive events detected in the test sample. The identity and/or quantity of the radionuclide can then be determined.
It is recognized that pulse shape may be indicative of various kinds of radioactive events. For example, a pulse of light detected by a liquid scintillation counter (LSC) can be classified as having been caused by either an emitted beta particle or an alpha particle based on its pulse shape. FIG. 1 is a plot showing normalized light intensity as a function of time. The alpha pulse has a longer tail (longer decay period) than the beta pulse. By using appropriate calibration samples according to the expected radionuclides in a test sample, a discriminator can be derived to classify a given radioactive event in the test sample as either alpha or beta radiation.
Similarly, U.S. Patent Application Publication US 2004/0262530 presents a technique for discrimination between pulses produced by neutrons and gamma rays based on pulse shape.
However, current discrimination techniques suffer from alpha/beta misclassification (spill), particularly when used in the detection of extremely low-level alpha and beta events, and when used to identify isotopes with difficult-to-distinguish pulse shapes, such as Strontium-90.
Thus, there is a need for improved systems and methods for discrimination between different kinds of radioactive events (e.g., alpha v. beta) in a test sample.